The problem of removal of sulfur dioxide from stack gases has been the subject of many proposals over the years. It has been recently reported (Chemical Week, Feb. 10, 1971) that sulfur dioxide emissions from power plants, smelters, acid plants, etc. amounts currently to 37 million long tons per year and is expected to reach 61 million long tons per year by 1980. It is obvious from these figures that past attempts to solve this problem, for a variety of reasons, have failed. In some instances, the methods are effective but are prohibitively expensive to build and/or to operate. For example, Karlsson, U.S. Pat. No. 3,436,192, operates on the principle of catalytic conversion of sulfur dioxide to sulfur trioxide and the subsequent conversion of the sulfur trioxide to sulfuric acid. Aside from the fact that the vanadium pentoxide catalyst is expensive and that substantial amounts of this catalyst are lost by attrition during regeneration, only about 85% of the sulfur dioxide will be removed; the catalyst requires preheating to elevated temperatures (i.e. 700-750.degree. F); the efficiency of the system is strongly upon upn optimum rate of plant operation and drops sharply when the plant load is other than at its optimum level; and the end product is 78 percent sulfuric acid which is too dilute for many industrial uses and is economically unsuitable for shipping any great distance.
Many other systems have been proposed in which chemical conversion of sulfur dioxide is involved. Fatlinger et al., U.S. Pat. No. 3,510,253, is an example in which stoichiometric amounts of ammonia are used to form ammonium sulfite or ammonium bisulfite which may be recovered and by reaction with a strong base converted into a corresponding other sulfite with liberation of ammonia for recycle purposes. Alternatively, the sulfite solution may be admixed with sulfuric acid to convert the sulfite to ammonium sulfate with simultaneous liberation of sulfur dioxide and the liquor recycled as a pre-stage system prior to the ammonia addition stage. In any case, an on-site sulfuric acid plant is required to convert sulfuric dioxide to the sulfuric acid used to displace sulfur dioxide from the scrubbing liquor or an independent sulfuric acid supply would be required should the end product be sulfur dioxide or ammonium sulfite.
The Fatlinger et al. patent represents but one approach in the chemical conversion area which, in general, involves the formation of a sulfite. For example, the Bevans et al. patent, U.S. Pat. No. 3,386,798, involves the conversion of calcium chloride to calcium sulfite and calcium sulfate; the Clarke patent, U.S. Pat. No. 2,128,027, involves the conversion of an organic base to the corresponding sulfite; and Lessing and Nonhebal et al., U.S. Pat. Nos. 2.080,779 and 2,090,142, respectively, both involve conversion of lime or limestone to calcium sulfite and calcium sulfate. In all of these processes, although it appears not to have been recognized, a serious limitation exists due to chemical equilibrium problems arising between the sulfur dioxide and the sulfite produced, i.e. a significant fraction of the sulfur dioxide is not absorbed because of the chemistry of the reaction even in a well engineered system.
That is to say, sulfur dioxide is readily expelled from sulfites by heat or by strong mineral acids. As a matter of fact, a standard laboratory procedure for preparation of sulfur dioxide involves heating an acidic aqueous solution of sodium bisulfite. Thus, prior art wet scrubbers which absorb sulfur dioxide with an acidic aqueous medium to form sulfites are propense to allowing substantial amount of sulfur dioxide to escape into the atmosphere. A number of systems employ an alkaline wet scrubber medium which successfully minimizes expulsion of sulfur dioxide by acid displacement and therefore overcomes this disadvantage of acidic mediums. However, alkaline wet scrubber mediums require a slurry form of medium containing an excess of alkaline earth material sufficient to neutralize the carbon dioxide as well as the sulfur dioxide in the effluent gas. This entails using an excess of reagent as well as adding to the engineering problems involved by the requirement for circulating a slurry.
Couillaud et al., U.S. Pat. No. 3,733,393, discloses the oxidation of sulfur dioxide by a concentrated solution of hydrogen peroxide; Hammond, U.S. Pat. No. 3,760,061, teaches that sulfur dioxide can be oxidized to sulfuric acid by contacting the sulfur dioxide with a solution of sulfuric acid and hydrogen peroxide; and Burrage, U.S. Pat. No. 2,165,784, teaches the removal of sulfur compounds from acidcontaining gases by contacting the gases with an aqueous solution of the acid containing a salt which will form an insoluble surface and the insoluble sulfate or a catalyst adapted to oxidize sulfites into sulfates.